1. Field of the Invention
The present invention concerns a fuel injection strut for a ramjet operating over a wide range of Mach numbers, especially at a high Mach number, for example in the order of 12 to 15.
2. Description of the Related Art
Ramjets are known to be particularly advantageous for propulsion of hypersonic aircraft (guided weapons, missiles, aircraft, etc) as they can operate over a wide range of Mach numbers, for example from 2 to 15, and have a low specific fuel consumption. Depending on the particular application of an aircraft, and possibly on the aircraft's flight phase, the fuel used can be a liquid hydrocarbon such as kerosene, for example, or a gas such as hydrogen or methane, for example.
A ramjet has, on the one hand, at least one inlet for combustion-supporting gas, usually consisting of an air cowling or an air intake, directing a flow of combustion-supporting gas (in particular air) towards a combustion chamber, and, on the other hand, at least one injection device for injecting the fuel into said flow of combustion-supporting gas so as to obtain a flow of fuel/combustion-supporting gas mixture that is ignited in said combustion chamber.
In ramjets designed to operate at a relatively low Mach number (for example up to Mach 2), a fuel injection device of the above kind can be made up of a set of individual injectors disposed on the inside wall of the ramjet at the periphery of the flow of combustion-supporting gas.
However, for operations at high Mach numbers, when combustion in the ramjet occurs in a supersonic or hypersonic flow, the fuel can no longer be injected only at the inside wall of the ramjet. In this case the penetration of the jets of fuel into the flow of combustion-supporting gas is too small for good mixing of the fuel and the combustion-supporting gas to be obtained within said flow, with the result that combustion is poor or even impossible. This problem is naturally aggravated if the transverse dimensions of the flow of combustion-supporting gas are larger.
To remedy this, injection devices have been designed in the form of manifolds with a plurality of individual injectors disposed along their length, which are disposed in and transversely to the flow of combustion-supporting gas, with the ends of said manifolds attached to opposite walls of said ramjet. An injection device of this kind is generally called an "injection strut" and is used either in isolation or in combination with injection of fuel at the wall.
With injection struts it is possible to obtain a satisfactory fuel/combustion-supporting gas mixture throughout the cross-section of the flow of combustion-supporting gas. More generally, injection struts in hypersonic ramjets:
assure a supply of fuel throughout the flow of combustion-supporting gas, despite the low penetration of the jets of fuel into a flow of combustion-supporting gas at hypersonic velocity; PA1 increase the proportion of fuel in the fuel/combustion-supporting gas mixture; PA1 assist ignition of the fuel/combustion-supporting gas mixture and stabilization of the flame; PA1 contribute to compression of the flow of combustion-supporting gas by slowing the flow of combustion-supporting gas scooped up by the ramjet. PA1 a chamber provided in the concave side of said permeable wall of the stem, and PA1 means for injecting a cooling fluid into said chamber, said injection means being adapted to generate a plurality of jets of a cooling fluid distributed along the height of said stem and impinging on the concave face of said permeable wall, at least in the region of said leading edge, so as to pass through said permeable wall.
Injection struts of the above kind, which are subject to the action of the flow of combustion-supporting gas, each behave, from the aerodynamic point of view, like a wing the ends of which are built into two opposite walls of the ramjet. On their stem side, opposite the fuel injectors disposed at the stern of the strut, which receives the flow of combustion-supporting gas, said injection struts must have a leading edge with a small radius to reduce pressure losses, which would limit the propulsive performance of the ramjet and could even lead to blockage of the flow of combustion-supporting gas, which can remain hypersonic within the combustion chamber only if the combustion-supporting gas velocity on its upstream side is sufficiently high.
However, heating of said stem caused by the hypersonic flow of combustion-supporting gas is substantially inversely proportional to the square root of the radius of the leading edge of said stem. Consequently a stem with a small radius leading edge is subject to intense heating. Note further that, said injection struts being disposed inside the ramjet, it is impossible to cool them by radiation with the air through which the aircraft propelled by said ramjet is flying. A stem of this kind is therefore subjected to very high temperatures, in the order of 5 000 K in the case of an aircraft flying at Mach 12 at an altitude in the order of 30 km. It is therefore necessary to construct the injection struts from materials such as ceramics, the radius of said leading edge being in the order of 3 to 5 mm. However, given current processes for manufacturing ceramic parts, it is evident that the fabrication of ceramic material injection struts, which must satisfy the requirement for high precision, is necessarily long and costly.